A background sheet conveying device apparatus of a first example includes a path selector disposed in a conveying path. When the trailing edge of a proceeding paper sheet passed the path selector, conveying rollers are switched to rotate in the opposite direction, and the trailing edge of the proceeding paper sheet is guided to a sheet stacking portion to store the paper sheet. Thereby, the proceeding paper sheet can be stacked with a following paper sheet to be conveyed together. In the background sheet conveying device, the above-described operation is repeated so that two or more paper sheets can stack to be conveyed as stacked paper sheets or a sheet stack.
A background sheet conveying device of a second example includes a recording sheet feeding section, a processing tray, a sheet detecting sensor, and a recording paper feeding control section. The recording sheet feeding section conveys paper sheets along a path to an outlet. The processing tray temporarily accumulates the paper sheets in the recording sheet feeding section. The sheet detecting sensor determines whether the paper sheets conveyed from the recording sheet feeding section has different types or different sizes. The recording paper feeding control section controls the number of paper sheets to be accumulated in the processing tray when the paper sheet conveyed from the recording sheet feeding section has different types or different sizes.
A background sheet conveying device of a third example includes a shift tray, a staple tray, a first carrying path, and a second carrying path. The shift tray directly stacks paper sheets discharged from an image forming apparatus or stacks sheet stacks after a sheet conveying process. The first carrying path runs from an inlet part to the shift tray. The second carrying path is branched from the first carrying path and runs toward the staple tray. A switching claw is provided at the first carrying path. An accumulation carrying path is branched from the first carrying path at the switching claw to merge the second carrying path.
In the background sheet conveying device of the first example, when the trailing edge of a paper sheet is guided to the sheet stacking portion for stacking, the leading edge of the paper sheet is held at the nip of conveying rollers extending therefrom by a specific amount of length. That is, the paper sheet is backwardly conveyed to the sheet stacking portion for stacking, is stopped at an appropriate position, and is forwardly conveyed immediately before the leading edge of a next paper sheet reaches the conveying rollers so that the amount of shift between the two paper sheets can be reduced when the two paper sheets are overlaid and conveyed. However, if the above-described operation is performed for paper sheets having different sizes, a paper sheet having a longer length in a sheet conveyance direction needs a longer distance to store the trailing edge, which takes a longer time to perform. Therefore, a longer interval between paper sheets is required.
Recent image forming apparatuses have a higher speed and longer life as well as shorter intervals of sheets. The background sheet conveying device cannot smoothly perform with such image forming apparatuses. For example, while a background image forming apparatus is performing a backward rotation of a long paper sheet, a recent image forming apparatus feeds a next paper sheet before the trailing edge of the long paper sheet reaches a reference position. This operation cannot successfully overlay the paper sheets to smoothly convey the paper sheets. Therefore, the intervals of paper sheets have to be increased, which can result in poor productivity of the image forming apparatus.
Further, the background sheet conveying devices of the second and third examples have not reduced the interval of sheets.